The present invention relates to apparatus for cleaning particulate contamination from the surfaces of web substrates; more particularly to cleaning apparatus having a plurality of particle-attracting (xe2x80x9ccontact cleaningxe2x80x9d) rollers for transferring particulate contamination from a web surface to the surfaces of the rollers; and most particularly, to such apparatus whereby both surfaces of a web may be cleaned simultaneously and wherein the rollers may be moved out of contact with the web as desired, and especially when the web is stationary.
It is well known to convey a web substrate around a particle-attracting roller to cause particulate contaminants on the surface of the web substrate to be transferred to the surface of the roller, thereby cleaning the web substrate. Such rollers are known in the art as xe2x80x9ccontact cleaning rollersxe2x80x9d (CCR""s), and may be provided with a tacky surface comprising, for example, adhesive tape as disclosed in U.S. Pat. No. 4,009,047, or an electrostatically-active polymeric surface, for example, a polyurethane as disclosed in U.S. Pat. Nos. 5,611,281 and 5,699,738.
The surface of a contact cleaning roller becomes progressively clogged with particles during use, reducing its effectiveness; therefore, the roller surface requires cleaning or xe2x80x9c renewalxe2x80x9dfrom time to time. Typically, renewal involves the use of liquid agents such as water or solvents to release the particles from the roller surface. Renewal typically leaves the surface of the roller wet for a short time. Experience has shown that if renewal is attempted while a CCR is in working contact with a substrate, the cleaning fluid can be tracked undesirably onto the substrate. Various multiple-roller schemes have been proposed to permit engaging a fresh CCR with a web and disengaging a particle-loaded CCR for cleaning while maintaining continuous cleaning of the web substrate. See, for example, the above-cited U.S. Pats., and also U.S. Pat. No. 5,251,348 (""348).
Patent ""348 discloses an axially rotatable turret disposed transversely of the conveyance path of a web substrate, the turret supporting three equidistantly-spaced contact cleaning rollers adjacent one surface of the web to be cleaned. In the rest position of the turret, two rollers are in rolling contact with the moving web, and the third roller is in a cleaning position for renewal. When the third roller has been cleaned and dried, it may be re-engaged with the web by rotating the turret 120xc2x0 to a new rest position, which also brings one of the other two rollers out of contact with the web and into position for cleaning. Thus, refreshed rollers may be continually supplied as needed without breaking continuity of cleaning contact of the web with one or more contact cleaning rollers.
The ""348 apparatus has been found to present a serious shortcoming when used to clean some delicate substrates, especially substrates having one or more fragile coatings on the surface being cleaned. When conveyance of the web is interrupted and the web is allowed to remain in stationary contact with the contact cleaning rollers for a period of time, the web surface can become adhered to the roller such that when conveyance is resumed the coating is stripped from the substrate and remains attached to the roller.
U.S. Pat. No. 5,855,037 discloses a scheme for preventing such adherence and stripping by periodically rotating the turret by a few degrees either clockwise or counterclockwise to bring new portions of the cleaning rollers and web into contact and to break contact between previously-contacting portions thereof before adherence can occur. Such periodic turret rotation can be operationally awkward as well as dangerous to operators, especially if it must be conducted every few seconds. Further, cleaning or change-out of the third roller during such a short rest interval may be impossible. In the extreme, web substrates which adhere very quickly or readily may not be cleaned safely using this apparatus, which cannot disengage all CCR""s from the web under any circumstances.
It is a principal object of the invention to provide improved web cleaning apparatus which can a) provide contact with a plurality of contact cleaning rollers during normal operation; b) provide easy cleaning or change-out of every contact cleaning roller in the apparatus; c) provide continuous contact, if desired, with at least one cleaning roller during all periods of web conveyance; d) disengage all contact cleaning rollers from the web whenever desired, whether the web is being conveyed or is stationary; and e) clean both surfaces of the web simultaneously.
It is a further object of the invention to provide improved web cleaning apparatus wherein the apparatus can be stopped mechanically and turned off electrically when fully disengaged from the web substrate.
Briefly described, a system for cleaning first and second surfaces of a web substrate being conveyed along a conveyance path includes first and second turret subsystems disposed sequentially along the path. Each turret is rotatable about an axis transverse to the direction of web conveyance but there is no axial cross-shaft. Each turret includes at least two contact cleaning rollers spaced apart in the turret. Preferably the rollers are disposed 180xc2x0 apart such that the axes of the rollers and the axis of the turret are contained in a plane. The web path proceeds through each pair of rollers such that in each turret the first roller of the pair is adjacent a first surface of the web and the second roller of the pair is adjacent a second surface of the web. When the turret is rotated sufficiently, neither of the rollers is in contact with the web (null position). When the turret is rotated to another position such that the plane forms an acute angle with the web path, preferably about 45xc2x0 or less, the web is engaged with the rollers on both sides of the web and follows a serpentine path through the turret.
In operation for web cleaning, at least one of the turrets is rotated to such a position that both rollers are engaged with the web. When it becomes necessary to renew one or both of the engaged rollers, the second turret may be rotated into engaged position such that both rollers in both turrets are engaged with the web. After the portion of the web between the turrets has passed through the downstream turret, the first turret may be rotated to disengage its rollers which then may be renewed and readied for re-engagement.
In a presently preferred embodiment, at least one of the turrets is provided with a second pair of contact cleaning rollers disposed in the turret in a second plane oriented at preferably 90xc2x0 from the plane containing the axes of the first rollers. In this embodiment, the spacing and diameters of the rollers are selected such that when the turret is rotated to a position in which both planes form an angle of 45xc2x0 to the web path, all four rollers are disengaged from the web substrate. From this position, when the turret is rotated through a suitable angle in either direction, for example, about 45xc2x0, one pair of rollers is fully engaged in cleaning position and the other pair of rollers is in renewal position. In this embodiment, the second turret assures continuity of cleaning as the first turret passes through the null position in rotating between a first cleaning position and a second cleaning position.
In a further preferred embodiment of the invention, each turret is provided with four rollers such that any pair of rollers in either turret may be rotated into renewal position at any time without breaking continuity of cleaning of the web surfaces.
In a further preferred embodiment of the invention, non-cleaning backing rollers are provided outside the turrets against which the contact cleaning rollers, in being rotated by the turrets, can nip the moving web, thereby increasing substantially the cleaning effectiveness of the contact cleaning rollers.